In a known manner, an ad hoc network has no fixed infrastructure. Radio communication stations, equipped with means for transmission and/or reception of radio signals and operating in accordance with appropriate protocols, form the nodes of the network and communicate with each other by means of a single radio channel or multiple shared radio channels.
An ad hoc network is in particular used to set in operation tactical communications between mobile military teams, for example, each provided with a radio communication station that constitutes a network node. Several teams are generally grouped together into groups, with the teams within a same given group being for example, part of a single hierarchical command structure.
There exist various different techniques for creating groups within radio communication networks.
A first known technique is referred to as “Multicast with IGMP Membership”. The IGMP (abbreviation for “Internet Group Management Protocol”) protocol used in IP (Internet Protocol) networks, allows a station to inform the stations it uses as relay stations, of its membership in one or more multicast groups.
Under this approach, the station, referred to as “source”, of the creator of the group sends a request to each member of the group that they have created, requesting that this member establish an IGMP membership affiliation with the multicast address that has been communicated in their request.
Consider that the user from a source station wants to create a group including the source station and a plurality of other stations.
The source station referenced as “ss”, includes an application block referenced as “s” in FIG. 1, for example the user's computer, and a radio frequency transmitting/receiving node, referenced as “ns.”
Only one of the plurality of other stations, referenced as “sd”, has been represented in FIG. 1. It also includes an application block referenced as “d” in FIG. 1, for example the users computer, and a radio frequency transmitting/receiving node, referenced as “na”. Depending on the topology of the ad hoc network at the instant in time concerned, it is considered that there is a radio frequency transmitting/receiving node relaying data between the originating source station and the other station and referenced as nr.
A method for creation of a multicast group with IGMP membership implemented relatively to the station ss and to the station sd is represented in FIG. 1.
A first step 10 takes place between the application block s and the node ns, wherein, following a command from the application block s to the node ns for creation of a group network address, this network address, called “@mid”, specific to the new group to be created, is provided to the application block s by the node ns: @mid=GetAddress( ).
Then the station ns, from the application block s, via the node ns, in a step 11, transmits a request to the station sd, via the relay node nr, and then via the node nd and forwarded to the application block d. This request reports the creation of a group to which the station sd belongs and requests for the station sd to establish an IGMP type membership affiliation with the group address indicated in the request.
This request is for example of the type JoinGroup(Gr Id, @d, @mid), where Gr Id is the name that identifies, at the application level, the group to be created with multicast network address @mid and @d is the address of the application block d.
Then, over the course of steps 12 13, 14, the implementation of the IGMP protocol makes it possible to set up membership affiliation associating the station sd, and then the relay node nr, with the multicast address @mid. Thus, in the step 12, an igmp(@mid, @d) message is transmitted to the node nd by the application block d. The latter thus informs the node nd that the address @mid is an address of a group to which the application block d belongs. This consequently results in all the messages sent to @mid having to be provided by the node nd to the application block d.
In the step 13, an igmp(@mid, @nd) message is transmitted to the relay node nr by the node nd. The latter thus informs the node nr that the messages sent to the address @mid should be transmitted to the node nd.
In the step 14, an igmp(@mid, @nr) message is transmitted to the node ns by the node nr. This latter thus informs the node ns that the messages sent to the address @mid should be transmitted by the node ns to the node nr.
The IGMP protocol thus creates a chain, or even a multicast tree, step by step, connecting the source and each member of the group. This chain or multicast tree, must be maintained by the network when the topology of the network is modified.
This process is in actual fact implemented relatively to the station ss and to each station of the plurality of other stations in the group.
Thus the establishment of a group according to this multicast with IGMP membership approach requires the sending of a message by the source to each member of the group, then the sending of membership affiliation messages over successive hops between each member and the source.
Thus for large sized groups (for example, with more than 50 members), the traffic load generated in the network and the time required for formation of the group, both of which depend on the number of members, are quite high.
Another known technique is called “Xcast”. The group is explicitly constituted by a membership affiliation request message from the originating source that cites all of the members of the group and is sent to each of these members. The exchanges of messages between the members of the group subsequently take place by using the explicit addresses of all the members. For a group with a large number of members, the size of the header, which includes the destination field, of each message exchanged, is thus fairly significant, which thereby increases the load supported on the network.
This present invention is aimed at providing a solution that makes it possible to reduce the disadvantages of the techniques of the prior art and in particular offers the ability to dynamically create and maintain one or more groups of stations while also minimising the signalling, the time and volume of exchanges necessary.